Mac Defender is not the first malware for Macs, but it may be the first to gain any serious traction. It is a rogue antimalware product which, when downloaded, demands payment for a security product to clean up purported infections and also uses the browser to display porn sites.

There have been many wolf-crying incidents over the years about Mac malware, especially from vendors with a vested interest in users wanting to buy it, but it is perfect plausible that the problem could develop in earnest. This could be it.